PROJECT SUMMARY In the United States, one in three adults will develop hypertension and require antihypertensive treatments in their lifetime. More importantly, only 50% of patients with hypertension have blood pressure controlled with current antihypertensive drugs. Poorly controlled hypertension is very prevalent in aging men and postmenopausal women with impaired pressure natriuresis response. The factors contributing to poorly controlled hypertension remain to be determined. It is clear, however, the development of all forms of hypertension will require continuously resetting of the pressure natriuresis response to higher pressures. Although previous studies have implicated the Na+/H+ exchanger 3 (NHE3) in the pressure natriuresis response and angiotensin II (ANG II)-dependent hypertension, whether NHE3 in the proximal tubules of the kidney is directly involved in the physiological pressure natriuresis response and its resetting in ANG II-induced hypertension has never been specifically investigated previously. We have strong preliminary evidence that conditional deletion of NHE3 selectively in the proximal tubules (PT-NHE3-KO), or pharmacological inhibition of NHE3 selectively in the kidney, primarily in the proximal tubules, with a novel orally absorbable NHE3 inhibitor significantly increases the pressure natriuresis response and attenuates ANG II- and salt-induced hypertension by restoring pressure natriuresis response in mice. In this revised proposal, we will test the hypotheses that NHE3 in the proximal tubules of the kidney is directly involved in the physiological pressure natriuresis response and its resetting in ANG II-induced hypertension, and that pharmacological inhibition of NHE3 selectively in the kidney, primarily in the proximal tubules, will attenuate hypertension in animal models of genetic-, ANG II-, aging-, and sex-associated hypertension by restoring pressure natriuresis responses. In Specific Aim 1, we will test whether deletion of NHE3 selectively in the proximal tubules will increase pressure natriuresis response and lower blood pressure by inhibiting proximal tubule Na+ reabsorption, whereas upregulation, via overexpression, of NHE3 selectively in the proximal tubules will impair and reset pressure natriuresis response to higher pressures by stimulating proximal tubule Na+ reabsorption and promoting salt sensitivity of blood pressure. In a revised Specific Aim 2, we will test whether NHE3 in the proximal tubules is directly involved in the physiological pressure natriuresis responses to acute saline volume expansion, natriuretic peptides, or activation of AT2 receptor/NO/cGMP signaling, and in the resetting of pressure natriuresis responses in ANG II-, L-NAME-, aging- and sex-associated hypertension. In Specific Aim 3, we will test whether inhibition of NHE3 selectively in the kidney, primarily in the proximal tubules, with an orally absorbable NHE3 inhibitor will attenuate genetic-, ANG II-, aging- and sex-associated hypertension by restoring pressure natriuresis responses. The successful outcomes of this proposal will help develop novel drugs to treat poorly controlled or resistant hypertension by selectively targeting NHE3 in the proximal tubules.